Relationship between Body Mass Index and Bone Turnover Markers in Girls with Idiopathic Central Precocious Puberty

Background This study aimed to determine the effect of body mass index (BMI) on bone turnover markers in girls with idiopathic central precocious puberty (ICPP) according to weight status at diagnosis. Methods Two hundred and eleven girls with ICPP were divided according to their weight status at diagnosis into three groups: normal weight, overweight, and obese. The serum levels of total procollagen type 1 N-terminal propeptide (P1NP), N-terminal midfragment of osteocalcin, β-C-terminal telopeptide of type 1 collagen, and some biochemical indicators were measured. Associations between variables were evaluated by multiple regression analysis. Results Serum P1NP concentrations were significantly different among groups (p < 0.001). No other significant differences were noted in N-terminal midfragment of osteocalcin and β-C-terminal telopeptide of type 1 collagen. BMI was associated with estradiol (r = 0.155, p < 0.05) and inversely associated with P1NP (r = −0.251, p < 0.01), luteinizing hormone peak (r = −0.334, p < 0.01), follicle-stimulating hormone peak (r = −0.215, p < 0.01), and luteinizing hormone/follicle-stimulating hormone peak (r = −0.284, p < 0.01). Multiple regression analysis of factors associated with BMI showed that it was correlated with P1NP, follicle-stimulating hormone base, and luteinizing hormone peak in the overweight group and the obese group. Conclusions Our findings showed that BMI was associated with P1NP, revealing the reduction of bone formation in overweight and obese girls with ICPP. During the diagnosis and treatment of girls with ICPP, attention should be paid to body weight and bone metabolism.


Introduction
With changes in lifestyle and diet, the incidence of obesity has risen sharply worldwide [1]. According to worldwide data from the Global Health Observatory, 18% of youth aged 5 to 19 years were overweight or obese in 2016 [2,3]. Evidence is mounting that obesity will decrease the age that puberty starts, thus making obesity a risk factor for precocious puberty [4]. Recently, concern has grown that obesity may negatively afect bone development in children. Evidence exists that links excess fat to bone-related diseases such as osteoporosis and skeletal fractures [5]. Both adipose and bone tissue come from the same progenitor mesenchymal cells, which are endocrine organs capable of producing hormones [5]. Te connection between adipose and bone tissue seems to cause confusion, stemming from the fact that obesity is both a protective state and a risk factor for osteoporosis [6]. Te most recent evidence indicates that increased body mass index (BMI) translates to higher bone mineral density (BMD); however, the parallel increase in adiposity alters bone-regulating hormones that can diminish bone quality [7].
Idiopathic central precocious puberty (ICPP) is the most common form of precocious puberty in girls, and it often leads to adverse efects on physical and psychological maturity [8]. Precocious puberty is a complex biological process of sexual development in which bones are observed to mature prematurely and rapidly. When abnormal hormones afect bone quality in children with precocious puberty, there may be a subsequent impact on their bone quality in adulthood, especially considering the potentially lower BMD and the risks of osteoporosis and fractures [9].
During the growth process, bone turnover markers (BTMs) may act as sensitive markers for dynamic observation of the bone turnover process and early detection of abnormal bone mass changes. Among such markers, procollagen type I N-terminal propeptide (P1NP) and Nterminal midfragment of osteocalcin (N-MID) refect bone formation, while β-C-terminal telopeptide of type 1 collagen (β-CTX) refects bone resorption [10]. Bone mineral mass changes are infuenced by various hormones, which directly or indirectly regulate bone physiology [11]. Teir levels may also provide information about bone metabolism [12,13]. A major determinant of osteoporosis is the magnitude of peak bone mass achieved in early adulthood. Peak bone mass, together with the bone size, geometry, and microstructure achieved in children and youth, constitutes total bone strength and resistance to fractures in the later stages of life [14][15][16]. Terefore, it is very important to focus on the changes in bone metabolism in children and adolescents diagnosed with ICPP.
However, there are few reports on how BMI afects BTMs in girls with ICPP. To understand whether BMI afects BTMs, we investigated the relationship between BMI and BTMs in 211 girls diagnosed with ICPP in our center. We hope that this analysis will help in the diagnosis and treatment decisions for girls with ICPP and improve the bone metabolism of girls with diferent levels of obesity.

Participants.
Two hundred and eleven girls diagnosed with ICPP aged 5-10 years participated in this study. Tey were recruited from the outpatient clinic unit of West China Second University Hospital from March 1, 2020, to October 31, 2021. All participants were confrmed to show signs of secondary sexual development, such as breast enlargement, before the age of 8 years, and none of the participants had received treatment. Te individuals were divided into groups according to BMI as follows: normal weight, 5-10 years (n � 89); overweight, 6-10 years (n � 70); and obesity, 5-9 years (n � 52). Te evaluation criteria for normal weight, overweight, and obesity were a BMI value between the 5th percentile and <85th percentile of normal girls of the same age, a BMI value between the 85th percentile and the <95th percentile of normal girls of the same age, and a BMI value for ≥95th percentile of normal girls of the same age, respectively [17]. Height and weight were measured by a nurse. Te height measurement did not include shoes, and the weight measurement did not include clothing and shoes. Tanner stage ratings 1 through 5 were assigned by clinicians. Tanner stage was used only for descriptive purposes and was not included in the models to adjust for growth/puberty. Instead, we used age to adjust for growth.

Inclusion Criteria.
All of the girls included in the study met the diagnostic criteria of ICPP [18]. Te diagnosis of ICPP requires the following conditions to be met simultaneously: (1) development of secondary sex characteristics before the age of 8 years in girls and 9 years in boys, with the same sequence of sexual development as that in children with normal sexual development; (2) a higher annual growth rate compared with children of the same age and sex; (3) higher bone age relative to the chronological age by at least 1 year; (4) increased uterus and ovaries, and more than one ovarian follicle with a diameter of ≥4 mm on pelvic ultrasound in girls; and (5) hypothalamic-pituitary-gonadal axis activation.
To determine whether hypothalamic-pituitary-gonadal axis function is activated, a gonadotropin-releasing hormone (GnRH) excitation test is required to achieve a luteinizing hormone (LH) peak ≥5.0 U/L and an LH/follicle-stimulating hormone (FSH) peak ratio >0.6. Participants with secondary obesity, such as hypercortisolism or primary thyroid obesity caused by low function, who had previously received hormone drugs or were receiving drugs for weight loss, diabetes mellitus, and other chronic liver and kidney diseases were excluded. Te study protocol was approved by the Clinical Trial Ethics Committee of West China Second University Hospital, Sichuan University (No. 2020040). Informed consent was obtained from the parents of all of the girls before enrollment. All clinical investigations were conducted in accordance with the principles outlined in the Declaration of Helsinki.

Collection of Blood Samples and Measurement of
Biomarkers. Te GnRH stimulation test was performed before diagnosis in all children, and fasting venous blood samples were collected before the GnRH test and 30 minutes, 60 minutes, 90 minutes, and 120 minutes after the start of the GnRH test. Blood samples were collected at 9:00 am after a fasting period of 8 hours for the detection of BTMs and biochemical indicators. Aliquots of 4 mL of whole blood were placed in a dry tube with no anticoagulant for serum separation. Serum aliquots were immediately stored at −80°C until further analysis. BTMs, including P1NP, N-MID, and β-CTX, were measured with a Cobas E411 electrochemical luminescence immunity analyzer (Roche, Mannheim, Germany). Serum insulin-like growth factor 1 (IGF1) and insulin-like growth factor binding protein-3 (IGFBP-3) levels were determined using an IMMULITE 2000 chemiluminescence analyzer (Siemens AG FWB, Berlin, Germany). An ADVIA Centaur XP system (Siemens AG FWB) was used to measure the serum levels of free triiodothyronine, free thyroxine, triiodothyronine, thyroxine, thyroid-stimulating hormone (TSH), LH, FSH, and estradiol (E2). Serum alkaline phosphatase, fasting blood glucose, homocysteine, total cholesterol, triglyceride, highdensity lipoprotein cholesterol, and low-density lipoprotein cholesterol were measured using an ADVIA 2400 system (Siemens AG FWB). Serum vitamin D was evaluated using an Abbott i2000SR immunoluminescence detector (Abbott Laboratories, Chicago, IL, USA). Te analytical variability of the BTMs, including intra-assay and interassay coefcients of variation, was all less than 8.0%. Te total coefcients of variation for E2, LH, and FSH were 1.9%-7.0%, 2.7%-3.8%, and 2.2%-3.9%, respectively.

Statistical Analysis.
Data were analyzed using SPSS software version 23.0 (IBM, Armonk, NY, USA). Descriptive statistics are presented as median and quartiles for nonnormally distributed characteristics. Comparison between groups was conducted using the Kruskal-Wallis H test. Additionally, Kolmogorov-Smirnov tests were used to compare any two groups. Chi-square tests were used for comparison of count data. Spearman correlation analysis was performed to examine the correlations among data. Associations between variables were evaluated using a multiple regression analysis. A value of p < 0.05 was considered signifcant for all the statistical analyses.

Baseline Characteristics.
For participants, the following clinical data were collected from the medical records, namely, chronological age, height, weight, and bone age at the time of diagnosis. A total of 211 girls with ICPP were included, of which girls with normal weight, overweight, and obesity accounted for 42.2%, 33.2%, and 24.6% of the sample, respectively. No diferences between groups were found in chronological age. Te normal weight group was shorter compared with the overweight group (p < 0.05) and the obese group (p < 0.05). Tere were signifcant diferences in weight between the normal weight group and the overweight and obese groups (p < 0.05). Te bone age in the obese group was more advanced than that in the normal weight group (p < 0.05) and the overweight group (p < 0.05) ( Table 1). Te number of participants stratifed by Tanner staging is shown in Table 1. All participants showed Tanner breast scores ≥2 before the age of 8 years. General characteristics of the participants are shown in Table 1, and general characteristics of ungrouped ICPP girls are shown in Supplementary Table 1.

Bone Turnover Markers Analysis.
Results from the analyses of BTMs are presented in Table 2. Te levels of P1NP measured in blood were signifcantly diferent among groups (p < 0.001). Levels of P1NP in the normal weight group were signifcantly higher than those in the overweight and obese groups (p < 0.05). P1NP levels in the obese group were signifcantly lower than those in the overweight group (p < 0.05). However, no other signifcant diferences were noted in N-MID and β-CTX. BTM levels for the participants are shown in Supplementary Table 1.

Blood Biochemical Characteristics.
Tere were signifcant diferences in IGF1, IGFBP-3, homocysteine, free triiodothyronine, free thyroxine, TSH, LH base, LH peak, FSH peak, and LH/FSH peak among the three groups (p < 0.05). We did not fnd specifc alterations of alkaline phosphatase, fasting blood glucose, thyroxine, high-density lipoprotein cholesterol, and FSH base concentrations in any group. Te serum levels of free thyroxine, TSH, vitamin D, LH peak, FSH peak, and LH/FSH peak were signifcantly diferent in the normal weight group compared with the overweight group. Serum IGF1, IGFBP-3, free triiodothyronine, triglyceride, low-density lipoprotein cholesterol, E2, LH peak, FSH peak, and LH/FSH peak levels in the normal weight group were also diferent from those in the obese group. In addition, we found that IGFBP-3, homocysteine, triiodothyronine, TSH, total cholesterol, triglyceride, E2, LH base, LH peak, and LH/FSH peak concentration were signifcantly diferent between the overweight group and obese group. Table 3 shows the diferences between any two groups of results. Te blood biochemical characteristics of participants are shown in Supplementary Table 1.

Regression Analysis of Factors Associated with BMI.
We performed a multiple regression analysis of the variables that are afected by BMI (Figure 1). BMI was correlated with P1NP, FSH base, and LH peak in the overweight and obese groups.

Te International Osteoporosis Foundation and the International Federation of Clinical Chemistry and Laboratory
Medicine strongly recommend β-CTX and P1NP as BTMs for clinical research. β-CTX in the circulation is greatly afected by circadian rhythm and food intake. Serum P1NP is not signifcantly afected by food but is afected by small circadian rhythms, drugs that stimulate bone growth, sex hormones, and glucocorticoid treatments. Terefore, we measured BTMs in the morning on an empty stomach. Evidence is inconsistent regarding the association between BMI and BTMs. Some studies have reported increases in BTMs in children and young adults with obesity [19,20], but others have reported reductions in BTMs with obesity [21]. Further studies have pointed out that the BTMs of children with obesity are signifcantly lower than those of children with normal weight [22]. Our results show that the levels of P1NP measured in blood were signifcantly diferent among groups, but no signifcant diferences were noted in  Signifcantly diferent from the overweight group. b Signifcantly diferent from the obese group. c Signifcantly diferent from the normal weight group. ALP, alkaline phosphatase; FBG, fasting blood glucose; IGF1, insulin-like growth factor 1; IGFBP-3, insulin-like growth factor binding protein-3; HCY, homocysteine; FT3, free triiodothyronine; FT4, free thyroxine; T3, triiodothyronine; T4, thyroxine; TSH, thyroid-stimulating hormone; TC, total cholesterol; TG, triglyceride; HDL-C, high-density lipoprotein cholesterol; LDL-C, low-density lipoprotein cholesterol; VitD, vitamin D; E2, estradiol; LH-base, luteinizing hormone base; FSH-base, follicular stimulating hormone base; LH-peak, luteinizing hormone peak; FSH-peak, follicular stimulating hormone peak; LH/FSH peak, luteinizing hormone/follicular stimulating hormone peak.  [23]. Second, the diet of children with obesity is usually high in fat, which may reduce intestinal calcium absorption, leading to lower calcium availability for bone formation [24]. Studies have shown that the adipose tissue of children with obesity expresses many adipokines, which can downregulate osteoblasts, osteocytes, and muscle cells and upregulate osteoclasts [25]. Our fndings show that regardless of which two groups were compared, no signifcant diferences were noted in β-CTX. Tis result seems to be inconsistent with the upregulation of osteoclasts. BMD is afected by many factors during the teenage years, and genetic factors account for 60-80% of BMD [26]. In addition, the hormonal milieu and individual and environmental modifable factors, such as body weight, obesity, diet, and physical activity, also play an important role in BMD later in life [27,28]. Terefore, the reason for this inconsistency cannot be simply attributed to a specifc reason, and further research is needed.
Our study shows that the LH peak of children with overweight and obesity was lower than that of children with normal weight, and the diference was statistically signifcant. Spearman correlation analysis shows that the LH peak and FSH peak are negatively correlated with BMI. Te multiple regression analysis also showed that BMI was signifcantly correlated with the FSH base and LH peak. Tis fnding is consistent with that of related research [29], but diferent from the fndings of Giabicani et al. [30]. Currently, the mechanisms underlying decrease in LH and FSH levels with increased BMI are unclear. Moreover, we observed an association between LH base with P1NP and FSH base, and FSH peak with N-MID. Previous reports on elderly individuals, adolescents, and collegiate athletes failed to observe any association between BTMs and FSH or LH [31,32]. One report on healthy Chinese females found that both FSH and LH were negatively related to bone mass [33]. Te discrepancy between our results and previous reports may be explained by diferences in ethnicity, age, and the impact of the disease.
Te present study has some limitations related to its cross-sectional design and small sample size. BTMs are afected by many factors, such as the region, movement, and nutritional characteristics. In future studies, we will conduct more experiments to verify whether weight has an efect on the reduction of bone formation in girls with ICPP. Moreover, there is still a lack of reference values for BTMs in healthy children in China. Terefore, our follow-up research will also establish a reference value for children's bone metabolism, and further analyze the factors that afect children's bone metabolism. Te exclusion of boys owing to the absence of available blood samples may have introduced bias. After the use of GnRH analogs to treat ICPP, there may be a disproportionate increase in BMI [34]. Terefore, it is also important to evaluate the correlation between BMI and bone metabolism in patients after GnRH treatment in future studies. Te present study also did not involve specifc segmentation of the Tanner staging of the girls because of the limited sample size. Terefore, further well-designed studies are warranted to confrm the present fndings.

Conclusion
Te main fnding of the present study is that BMI is associated with P1NP, which reveals the reduction of bone formation in overweight and obese girls with ICPP. Obesity may be related to bone metabolism in girls with ICPP. Overall, when diagnosing precocious puberty, the status of patients with obesity should be considered as an infuencing factor of puberty hormone concentration, and the degree of  International Journal of Clinical Practice obesity should be evaluated when assessing precocious puberty. Being overweight and obese may adversely infuence bone formation in girls with ICPP. In the treatment process for ICPP in girls, more attention should be paid to weight management.

Data Availability
Te data used to support the fndings of this study are included in the article. Due to the sensitivity of the data and the restrictions from the informed consent, the data will not be stored at a public repository but are available from the corresponding author upon reasonable request.

Additional Points
What Is Known? Idiopathic central precocious puberty (ICPP) results from premature activation of the hypothalamic-pituitary-gonadal axis before the age of 8 years in girls and 9 years in boys. Obesity is a risk factor for precocious puberty and body mass index (BMI) is associated with bone mineral density and bone quality. Tere are few reports on how BMI afects bone turnover markers (BTMs) in girls with ICPP. What Is New? Te levels of total procollagen type 1 Nterminal propeptide (P1NP) were signifcantly diferent among normal weight group, overweight group, and obese group. BMI was associated with P1NP, revealing the reduction of bone formation in overweight and obese girls with ICPP. During the diagnosis and treatment of girls with ICPP, attention should be paid to body weight and bone metabolism.

Ethical Approval
Tis case-control study complied with the ethics of the World Medical Association Declaration of Helsinki and was approved by the Ethics Committee of West China Second University Hospital (Ethical Approval No. 2020040).